As electronic devices proliferate and diminish in size while simultaneously offering increased capabilities, there is concomitant need for enhanced integrated circuits, internal components and power supply systems. Demanding electronic systems of all stripes require finer and finer metal powders for multiple layer ceramic capacitors (MLCC's), batteries, switches, logic circuit components, etc.
Metal nanopowders, particularly ultrafine nickel powders, are produced in various ways. Chemical vapor deposition (“CVD”) techniques based on carbonyl technology provide especially pure and desirable chemical and physical characteristics. However, due to the relatively low operating temperature of conventional carbonyl powder decomposers (400-700° C.), the morphology of conventional carbonyl powders may not be sufficiently spherical and smooth. Hot wall carbonyl decomposers, although capable of producing ultrafine metal powders on the order of one micron or less, typically create spiky and irregular shaped powder particles. Nickel chloride based CVD techniques produce smoother particles but they run hotter, introduce environmental issues and are inherently more costly.
Equipment manufacturers employing ultrafine nickel powers, for instance, are increasingly demanding highly spherical particle morphologies and smooth surfaces to minimize surface area and thus inherent powder reactivity and to improve the particle packing density.
Researchers have been investigating the use of various types of carbonyl based systems to produce metal nanopowders.
U.S. Pat. No. 4,808,216 to Kageyama et al. discloses a gas phase pyrolysis process for producing ultrafine powders wherein a hot diluted carbonyl compound is passed through a strong magnetic field.
U.S. Pat. No. 5,403,375 to König et al. discloses a furnace that employs a plurality of gas streams to prevent the deposition of the selected powders onto the hot walls. Gaseous metal compounds are evaporated prior to their introduction into the furnaces.
U.S. Pat. No. 6,689,192 B1 to Phillips et al. discloses the introduction of a plasma gas into a microwave cavity.
Others have introduced solid nickel particles into DC spray plasma reactors, transferred arc plasma reactors and induction plasma reactors.
Indeed, one of the present co-inventors (M. Boulos) is an inventor of U.S. Pat. No. 5,200,595, incorporated herein by reference, that discloses a high performance induction plasma torch commercially available from Tekna Plasma Systems, Inc. of Sherbrook, Quebec Canada.
As noted above, parties have used inductively coupled radio frequency (“RF”) plasma systems to produce nickel nanoparticles by introducing fine nickel particles into the plasma. These nickel particles melt and vaporize in the plasma. As they exit the plasma, the gaseous nickel atoms condense into liquid droplets. The droplets cool and solidify into generally spherical nickel particles.
Plasma based processes for producing metal nanopowders are successfully utilized. However, they suffer from a number of drawbacks.
There are a number of disadvantages in using solid nickel feeds with DC arc and electrode based plasma reactors.
The reaction temperature using metallic nickel (or any metal for that matter) must exceed the melting point of the metal, which is 1453° C. for nickel. Since high plasma power is necessary, throughputs are limited. Metallic feeds, even if classified to fine dimensions, still contain oversize particles that will often pass through the plasma without being vaporized. These large particles report to the final product as undesirable size fractions.
Moreover, the use of metallic nickel feed requires a powder feeder. Powder feeds which are supposed to meter discrete quantities and rates of particles tend to both plug up and vary the feed rates causing unsteady reactor operations.
Electrode based plasma reactors such as DC spray plasma and transferred arc plasma systems introduce undesirable contaminants into the resulting powders from the electrodes.
There is needed an expeditious plasma method for making ultrafine metal spherical powders in general and nano sized nickel powders in particular.